(1) Field of the Invention
The present invention relates to an apparatus for controlling an amount of fuel-vapor purged from an adsorbent filled canister in a fuel-vapor emission-control system.
(2) Description of the Related Art
Generally, modern automobiles are equipped with an evaporative emission-control systems having a canister filled with an adsorbent such as activated charcoal, for capturing fuel-vapor from a fuel tank and preventing an escape thereof into the open air. Fuel-vapor is caused by evaporation, and a large part of the atmosphere in the fuel tank is composed of fuel-vapor. In the fuel-vapor emission-control system, fuel-vapor from the fuel tank flows to the charcoal canister, the charcoal particles pick up and hold the fuel-vapor, and, when the engine runs, air flows through the charcoal canister on the way to the intake air system, e.g., intake air pipe. This air picks up the fuel-vapor trapped in the canister and carries it to the intake air pipe, where it is mixed with the air-fuel mixture and fed to the engine and thus burned, instead of being allowed to enter the atmosphere as fuel-vapor.
In this fuel-vapor emission-control system, large quantities of fuel-vapor occur not only during a supply of fuel to the fuel tank but also just after this fuel supply is stopped. Accordingly, large quantities of vapor-laden air from the fuel tank are carried through the emission-control line and into the canister, where the fuel-vapor is adsorbed by the charcoal. In this context adsorbed is used to denote that the fuel-vapor is trapped by the charcoal particles. Therefore, the canister captures much fuel-vapor during and just after the fuel supply to the tank.
Note, when the engine is started for the first time after a fuel supply, fresh air is drawn in by the intake-manifold vacuum, is sent through the canister, and removes, or purges, a large amount of the fuel-vapor from the canister, even though an amount of air flow is the same as usual. If the adsorbent in the canister is an activated charcoal, the purge rate from the canister after the fuel supply to the fuel tank is stopped is high at first and then gradually decreases as shown in FIG. 1
As a result, when the vehicle is first run after a fuel supply, a large quantity of fuel-vapor is supplied into the intake air system, and it is likely that the amount of fuel-vapor supplied will exceed the control range of the air-fuel ratio controller. In this case, the air-fuel ratio becomes overrich and both the emission characteristic and the driveability of the vehicle will be worsened.